The present invention relates to a video tape recorder (VTR) or a video cassette recorder (VCR) which converts a luminance signal of a color television signal to an FM signal and a chrominance signal to a low frequency carrier for recording, and more particularly to a color signal processing circuit for a VTR and a frequency divider suitable for use therewith.
In a helical scan type video tape recorder having a rotary head, a jitter component may be superimposed on the reproduced signal due to an error in the video tape feed speed. In the video tape recorder which converts the color signal to a low frequency for recording, such a jitter component causes a variation of the frequency of the reproduced color carrier wave, which results in a shift of the color phase.
FIG. 1 shows one example of a prior art color signal reproducing circuit in a VHS (video home system) video tape recorder, which has been used as means to compensate for the jitter component which causes the shift of the color phase. The jitter component compensation means used in the video tape recorder is now explained with reference to FIG. 1.
In a playback mode, a signal reproduced by a playback head, not shown, from a video tape is supplied to an input terminal 1 which is connected to a low-pass filter (LPF) 4. A reproduced low frequency converted color signal taken out of the LPF 4 is reconverted to a color signal of an original sub-carrier band by a frequency converter 5 and it is supplied through a bandpass filter (BPF) 6 and a comb filter 7, which eliminates cross-talk components from adjacent video tracks, to an output terminal 3. The frequency of the color signal supplied to the output terminal 3 is represented by (f.sub.sc +f.sub.r)-(f.sub.r +.DELTA.f)=f.sub.sc -.DELTA.f, where f.sub.r is the frequency of the low frequency converted color signal, f.sub.sc is the frequency of the color signal in the color sub-carrier band (f.sub.sc is 3.579545 MHz for the NTSC system) and f.sub.sc +f.sub.r is the frequency of the converted carrier from a BPF 14. Thus, since the low frequency converted color signal f.sub.r from the LPF 4 includes the jitter component .DELTA.f, the color signal at the output terminal 3 includes the jitter component .DELTA.f. By superimposing the jitter component .DELTA.F on the converted carrier from the BPF 14 to change the frequency thereof to f.sub.sc +f.sub.r +.DELTA.f, the frequency of the color signal is (f.sub.sc +f.sub.r +.DELTA.f)-(f.sub.r +.DELTA.f)=f.sub.sc and the jitter component .DELTA.f is compensated.
In the above example, the jitter component .DELTA.f is compensated by a phase locked loop (PLL), particularly by an automatic phase control (APC) including at least the first frequency converter 5, the BPF 6, a phase detector 9, a voltage controlled oscillator (VCO) 10, a frequency divider 11, a phase shifter 12, a second frequency converter 13 and the BPF 14. A burst signal (f.sub.sc +.DELTA.f) of the color signal at the output terminal 3 which includes the jitter component .DELTA.f and an output signal of an oscillator 8 for the sub-carrier of the frequency f.sub.sc are phase detected, and an oscillation frequency of the VCO 10 having a center frequency of approximately 4 f.sub.r (f.sub.r =40 f.sub.H for the VHS system and f.sub.H =(44-1/4)f.sub.H the .beta. system, where f.sub.H is a horizontal scan frequency) is controlled by the phase detection signal. Assuming that the oscillation frequency f.sub.VCO of the VCO 10 is 4(f.sub.r +.DELTA.f), it is frequency divided by a factor of four by the frequency divider 11, an output of which is supplied to the frequency converter 13 and the sum frequency f.sub.sc +(f.sub.r +.DELTA.f) of the output signal from the frequency converter 13 and the output signal from the oscillator 8 having the frequency of f.sub.sc is taken out as the converted carrier frequency. The converted carrier is supplied to the first frequency converter 5 to compensate for the jitter component as described above.
However, a compensation range for the jitter component .DELTA.f by the APC of the above example is .+-.f.sub.H /2 for the NTSC system. If .DELTA.f=f.sub.H, the APC is stabilized when the oscillation frequency of the VCO 10 is 4 (f.sub.r +f.sub.H) and the sub-carrier frequency of the color signal supplied to the output terminal 3 is f.sub.sc +f.sub.H. Accordingly, the system is not exactly pulled into f.sub.sc. In order to prevent the stabilization of the system in an error state, a frequency discriminator 15 is provided in the above example. The frequency discriminator 15 controls the VCO 10 when the jitter component .DELTA.f is larger than .+-.f.sub.H /2 to prevent the deviation of the output of the VCO 10 from 4 f.sub.r .+-.2 f.sub.H.
FIG. 2 shows an example of the frequency discriminator 15. The examples shown in FIGS. 1 and 2 are for the VHS system and the frequency f.sub.VCO of the VCO 10 is selected to be 160 f.sub.H. The frequency discriminator 15 comprises a gate circuit 16, a frequency divider 26 which divides an H pulse of 1 H period (where H is one horizontal scan period) from an input terminal 2, a frequency counter including frequency dividers 17-23 and gate circuits 24 and 25, and a decoder 27. In operation, the H pulse from the input terminal 2 is frequency divided by a factor of eight by the frequency divider 26 to produce a pulse having a period of 8 H and a duty factor of 50%, which pulse is supplied to the gate circuit 16 so that the output pulses from the VCO 10 having the center frequency of approximately 160 f.sub.H are counted only for 4 H period of the 8 H period, and if the pulse count is 640.+-.1, the decoder 27 produces no control signal, if it is no more than 638 or no less than 642, the decoder 27 produces a control signal to the VCO 10 to increase or decrease the frequency of the VCO 10, respectively. An example of the control characteristic of the frequency discriminator 15 is shown in FIG. 3a. An example of conversion of the control characteristic to the oscillation frequency f.sub.VCO of the VCO 10 is shown in FIG. 3b. When the oscillation frequency f.sub.VCO of the VCO 10 is within the center frequency 160 f.sub.H .+-.f.sub.H /2, the control signal to control the oscillation frequency f.sub.VCO of the VCO 10 is not produced, and when the oscillation frequency f.sub.VCO deviates by more than .+-.f.sub.H /2, the control signal of appropriate polarity is produced by the decoder 27 so that the frequency deviation of the VCO 10 is always pulled into the range of .+-.f.sub.H /2.
The reason for selecting the pull-in range of the oscillation frequency of the VCO 10 by the frequency discriminator 15 to be .+-.f.sub.H /2 is to allow the frequency discriminator 15 to be used for both the NTSC system and the CCIR (Comit'e Consultatif International des Radio Communication) system e.g., the PAL system.
The compensation range for the jitter component .DELTA.f by the APC is .+-.f.sub.H /2 for the NTSC system as described above, while it is .+-.f.sub.H /4 for the PAL system. In actual practice, however, the compensation range imposed upon the APC should be smaller than that described above. Assuming that a margin for the compensation range is, for example, more than 6 dB, the compensation range by the APC should be set to less than .+-.f.sub.H /4 for the NTSC system and less than .+-.f.sub.H /8 for the PAL system. To convert them to the deviations of the oscillation frequency f.sub.VCO of the VCO 10, it is .+-.f.sub.H for the NTSC system and .+-.f.sub.H /2 for the PAL system. Accordingly, the pull-in range of the oscillation frequency f.sub.VCO of the VCO 10 by the frequency discriminator 15 should be less than .+-.f.sub.H for the NTSC system and less than .+-.f.sub.H /2 for the PAL system. Thus, in order to allow the frequency discriminator 15 to be used for both the NTSC system and the PAL system, the pull-in range of the VCO 10 is selected to be that of the PAL system, that is, .+-.f.sub.H /2.
In the video tape recorder, the carrier frequency of the low frequency converted signal for the NTSC system is different from that for the PAL system. Accordingly, the frequency f.sub.r +.DELTA.f corresponding to the low frequency converted carrier frequency of the converted carrier frequency f.sub.sc +(f.sub.r +.DELTA.f) should be differently set for the NTSC system and the PAL system. This is necessary in order to visually reduce a beat disturbance appearing in the luminance signal due to a secondary distortion component 2 f.sub.r of the low frequency converted color signal f.sub.r produced by the tape/head. The beat disturbance can be visually reduced by imparting an offset of f.sub.H /4 to the low frequency converted color sub-carrier frequency f.sub.r for the NTSC system, and an offset of 3 f.sub.H /8 for the PAL system.
In the present VHS system, for the NTSC system, the output signal from the VCO 10 of FIG. 1 is frequency divided by a factor of four to produce a signal 40 f.sub.H, which is phase shifted by the phase shifter 12 by .pi./2 for each 1H so that the offset of f.sub.H /4 is equivalently imparted to the signal supplied from the phase shifter 12 to the second frequency converter 13 in order to provide the offset of f.sub.H /4 to the low frequency converted color sub-carrier in the record mode. For the PAL system, an oscillator (not shown) having an oscillation frequency offset by f.sub.H /8 is provided in addition to the oscillator 8 of the frequency f.sub.sc and it is connected to the frequency converter 13 in place of the oscillator 8 of the frequency f.sub.sc, and the oscillator 8 is connected to the phase detector 9 as shown in FIG. 1 so that the offset of f.sub.H /8 is imparted to the component f.sub.sc of the converted carrier in order to impart the offset of f.sub.H /8 to the low frequency converted color sub-carrier in the record mode. In this manner, the component f.sub.r +.DELTA.f is differently set for the NTSC system and the PAL system in order to allow the selection of the same center frequency of the VCO 10 for both systems.
Thus, the low frequency converted color sub-carriers for the respective systems are different from each other by at least the offset amounts for the respective systems. However, since the offset is formed by other than the PLL which includes the frequency discriminator 15 and the VCO 10, the same center frequency of the VCO 10 can be used for the NTSC system and the PAL system. Accordingly, by selecting the pull-in range of the oscillation frequency of the VCO 10 to be that for the PAL system, that is, .+-.f.sub.H /2, the frequency discriminator 15 can be used for both the NTSC system and the PAL system, as described above.
However, the prior art example requires two oscillators of the frequency of approximately fsc for the CCIR system in order to allow the use of the frequency discriminator for both systems, and hence the cost substantially increases. If the number of oscillators for the PAL system is reduced to one in order to suppress the increase of cost, the oscillation frequency of the VCO 10 should be set differently for the NTSC system and the PAL system. In this case, the frequency discriminator cannot be used in common for both the NTSC and PAL systems. If the center frequency of the VCO 10 for the PAL system is set to be f.sub.H /2 higher than that for the NTSC system, and if the pull-in range of the frequency discriminator 15 for the NTSC system is selected as shown in FIG. 3b, the pull-in range for the PAL system is that shown in FIG. 3c, in which the center frequency of the VCO 10 is (160+1/2) f.sub.H. As a result, the pull-in range of the oscillation frequency of the VCO 10 cannot be within .+-.f.sub.H /2. Thus, the jitter component cannot be correctly compensated in the PAL system and a color phase shift appears on a reproduced image.
In a video tape recorder which converts a luminance signal to an FM signal and low frequency converts a color signal and frequency-multiplexes it on a low frequency side of the FM signal and overwrites the signals for each field by two heads having different azimuth angles, cross-talk components of the color signals from adjacent video tracks raise a problem because an azimuth effect is not attained for the low frequency color signal. To resolve the above problem, the prior art NTSC video tape recorder imparts an offset of f.sub.H /2 to the low frequency converted color signal between the fields in the record mode and uses a comb filter for the color signal in the playback mode.
The approach to resolve the cross-talk problem is now explained.
In the .beta. system video tape recorder, a phase invert (PI) system is adopted in which the low frequency converted color signal frequency is selected to be (44-1/4) f.sub.H and the low frequency converted color signal of only one field is phase inverted by 180 degrees for every horizontal scan period (1H). By imparting the offset of f.sub.H /2 to the low frequency converted color signals of adjacent fields, the cross-talk components from the adjacent tracks have the offsets of f.sub.H /2 to the color signal from the main track upon reproduction. Accordingly, the cross-talk components are suppressed by passing the color signal including the cross-talk components through the comb filter having a comb valley frequency offset by (N+1/N) f.sub.H (where N is an integer) with respect to a center frequency of the color sub-carrier.
However, a problem arises in imparting the offset of f.sub.H /2. FIG. 4 shows an example of the .beta. system color signal processing circuit. The problem encountered in the prior art now will be explained with reference to FIG. 4, in which numeral 201 denotes an input terminal for a color signal, numeral 202 denotes an input terminal for a synchronizing signal or a signal synchronized thereto, numeral 203 denotes an input terminal for a pulse which is phase inverted for every 1H, numeral 204 denotes an input terminal for a track address signal, numeral 205 denotes an output terminal for a low frequency converted color signal, numeral 206 denotes a voltage controlled oscillator (VCO) having an oscillation frequency of 175 f.sub.H, numeral 207 denotes a one-to-five frequency divider, numeral 208 denotes a one-to-thirty-five frequency divider, numeral 209 denotes a phase comparator for comparing a phase of the frequency-divided output of the VCO 206 and a phase of the horizontal synchronizing signal from the input terminal 202, numeral 210 denotes a one-to-four frequency divider, numeral 211 denotes a second frequency converter, numeral 212 denotes a band-pass filter (BPF) for extracting a sum frequency from the frequency converter 211, numeral 213 denotes a circuit which is non-operative in a first track and reverses phase for every 1H in a second track, numeral 214 denotes a first frequency converter, numeral 215 denotes a low-pass filter (LPF) for extracting a difference frequency signal from the frequency converter 214, numeral 216 denotes a 3.58 MHz crystal VCO, and numeral 217 denotes a phase comparator for comparing the phase of the signal from the crystal VCO 216 and the phase of a burst signal from a burst gate circuit 218. Thus, a low frequency carrier of (44-1/4) f.sub.H synchronized with the horizontal synchronizing signal is produced at the output terminal of the one-to-four frequency divider 210, and a carrier having the same frequency as the color signal applied to the input terminal 201 is produced at the output terminal of the crystal VCO 216. Those two carriers are supplied to the first frequency converter 211 so that a signal of (3.58 MHz+175/4 f.sub.H) is extracted at the output terminal of the BPF 212, and a signal of (3.58 MHz+175/4 f.sub.H) for the first track and a signal of {3.58 MHz+(175/4 .+-.1/2) f.sub.H } for the second track are produced at the output terminal of the phase inverter 213. Accordingly, the color signal converted down to 175/4 f.sub.H is produced at the output terminal 205 in the first track period, and the color signal converted down to (175/4 .+-.1/2) f.sub.H is produced in the second track period. In this manner, the low frequency converted color signals offset by f.sub.H /2 between the tracks (or fields) are produced.
However, since this circuit includes the phase inverter 213 succeeding the second frequency converter 211, the frequency of a signal subjected to phase inversion is high, that is, approximately 4.27 MHz and hence it is difficult to secure the phase reversal angle of 180 degrees. The shift of the phase reversal angle causes the degradation of the cross-talk suppression effect on the color signal. If it shifts by five degrees, the cross-talk suppression is approximately 15 dB at maximum and the quality of the reproduced image is materially degraded.
In addition, since the phase inverter 213 is inserted following the BPF 212, it is difficult to incorporate the phase inverter 213 in an IC module when the color signal processing circuit is to be constructed in the IC module and hence the number of peripheral parts increases.
The prior art described above is disclosed in an article "New Chrominance Signal Processing LSI for Home VCR" by Nakagawa et al, IEEE Transactions on Consumer Electronics, Vol. CE-26, August 1980, pages 315-322.